The calibration of an X-ray machine is important in diagnostic radiology. The measurement of the potential applied to an X-ray machine has been recognized as an important variable in the production of high quality diagnostic X-ray films. In the United States, the Radiation Control for Healthy and Safety Act of 1968 became law in 1973. The main intent of the law was to protect the population from unnecessary radiation exposure. One way to accomplish this is to reduce the number of retakes of X-rays. The law requires that X-ray machines meet certain requirements. One of these requirements is that the maximum applied input voltage, sometimes referred to as the peak kilovoltage (kVp), applied to the X-ray machine fall within certain limits specified by the manufacturer. If an X-ray machine is inaccurately calibrated, this may result in shortened component life and poor quality X-rays, which may result in retakes. Consequently, there is a need to periodically check the accuracy of the kVp setting on X-ray machines and recalibrate when required.
Diagnostic X-ray machines operate at relatively high voltages, such as on the order of 50 kV to 150 kV. Direct measurement of such a high voltage may be dangerous and has in the past been accomplished by disconnecting the high voltage circuits and reconnecting a high voltage divider having two large value resistance sections connected between the anode of the X-ray generator and ground and between the cathode of the generator and ground. The high voltage divider circuit is typically large in volume and size and the operation for measuring the high voltage in such apparatus is time-consuming and only qualified service personnel could accomplish this task. Hospital staff people have not normally been employed for conducting this test because of the size and weight of the divider circuit and the inherent danger involved in making such a measurement.
Alternatives to the direct measurement, utilizing a high voltage divider as discussed above, are various noninvasive measurement techniques presently being employed. This includes the use of a noninvasive film cassette, as well as a noninvasive electronic device employing filters and sensors. These noninvasive techniques measure the input voltage to an X-ray machine from measurements of the radiation the machine emits.
The film test cassettes (sometimes known as the Adrian Crooks or Wisconsin test cassette) have been used to determine the input kilovoltage to a radiation source from the measurements of the radiation it emits. A test cassette is placed in the field of an X-ray beam and operates on the principle that the extent of attenuation of an X-ray in a material, such as copper or aluminum, is related to the kilovoltage applied to the X-ray tube. X-ray film is exposed to X-rays that have been attenuated while passing through multiple layers of material including a copper sheet and a sheet that includes copper disks and holes. The measurement requires the assistance of skilled technicians, development of the film and reading of the film with a densitometer. The accuracy of this method is on the order of .+-.5 kV. Moreover, since such a test cassette can measure only the effective or average kV and not the true peak of the waveform, results will not reveal significant ripple or spiking on the waveform.
Another noninvasive device for measuring input voltage supplied to an X-ray machine takes the form of an instrument known in the art as a kVp meter. Examples of such meters are disclosed in various U.S. patents, including the patents to Zarnstorff et al., U.S. Pat. Nos. 4,697,280, Siedband, 4,361,900, as well as products manufactured by Keithley Instruments, Inc. as model Nos. 35070 and 35080. In general, these kVp meters operate on the principle of passing an X-ray beam through a pair of copper filters positioned side-by-side so that the X-ray beam is attenuated as it passes through each filter. The two filters are of different thicknesses and, hence, as the radiation passes through each filter, it is attenuated differently. The attenuated radiation from each filter is then detected by a pair of X-ray detectors, such as solid state photodiodes, which provide output electrical signals having magnitudes which depend upon the attenuated radiation levels from the two filters. A ratio of these two signals is then made. This ratio will vary with the input kilovoltage applied to the X-ray tube. The X-rays passing through the thicker material increase faster with increasing input kilovoltage than the X-rays passing through the thinner material. Consequently, the ratio of the signals representative of radiation passed through the thick material to that of the thin material starts at zero and increases as the kilovoltage increases. For very large kilovolts, the ratio approaches unity.
These kVp meters typically operate over a voltage range from 50 to 150 kV. This is known in the art as the diagnostic range. The ratio of the radiation passed by the thick filter to that of the radiation passed by the thin filter is used as a measure of the input kilovoltage. The linear range of this relationship is limited. For example, the Keithly Model No. 35080 kVp meter employs three sets of copper filters each of which has substantial linearity over a portion of the diagnostic range. Thus, one filter set is typically employed from 50 to 90 kV, a second filter set is employed for 65 to 135 kV, and a third filter set is employed from 75 to 150 kV. It would be preferable to employ a single set of filters which would have acceptable linearity throughout the entire diagnostic range from 50 to 150 kV.
In addition to the limited linearity of the relationship between the ratio and the magnitude of the input kilovoltage another problem is presented if a single pair of copper filters is employed to cover the entire diagnostic range of, for example, 50 to 150 kV. This problem deals with the limited dynamic range presented. That is, in order to obtain adequate signals for low voltages on the order of 50 kV to 90 kV, the copper filters must be made of relatively thin material. However, if the filters are too thin then the ratio displays too large a dependency on changes in the filtration of the X-ray generator at higher voltages. It would be desirable to provide a single filter set which has a dynamic range so that it is useful over the entire diagnostic range from, for example, 50 kV to 150 kV.
Attempts to increase the useful range of operation of such kVp meters as discussed above have included employing multiple filter pairs with each pair being assigned for use over a particular voltage range, as discussed above, or employing a plurality of filter pairs which are simultaneously exposed in the same instrument. Where a single pair of filters has been employed, it has been attempted to linearize the output signal electronically while tolerating the problems of the limited dynamic range. Consequently, some kVp meters cannot measure low voltage fluoroscopic signals satisfactorily while others have too much dependency on X-ray machine filtration.
The present invention is directed toward determining the operating voltage of an X-ray machine employing a single pair of filters having a useful range, both linear and dynamic, which covers the voltage range of interest. In the discussion given herein, the useful range of a single filter set may cover the diagnostic range of from 40 kV to 150 kV.
The present invention is based on the recognition that a chemical element, such as lead or gadolinium, exhibits an absorption phenomena. Such elements when irradiated by an X-ray beam will absorb radiation at a predictable rate until the voltage applied to the X-ray machine attains a particular level and then a sudden transition takes place in the absorption rate. This transition is a sharp increase in the absorption rate and it corresponds with what is known as the K absorption edge of that particular chemical element. The K absorption edge refers to the K quantum shell. An electron can be removed from the K shell by photoelectric absorption. This takes place when photons of a sufficiently high energy level are incident upon an atom causing an electron to be ejected from the K shell. The threshold photon energy to achieve this is known as the K absorption edge.
The patent to G. R. Harris et al. U.S. Pat. No. 3,766,383 discloses an apparatus for calibrating the kilovoltage of a diagnostic X-ray generator. By placing a chemical element or test sample, having a known K-absorption edge, within an X-ray beam. Harris does not propose a kVp meter as discussed above employing a pair of filters but only a single chemical element having a known K absorption edge. The chemical element or test sample is disposed at an angle of approximately 45 degrees to the generator radiation path so that some energy is reflected as scattered energy, and some energy is transmitted through the sample as transmitted energy. The scattered energy and transmitted energy are detected and a ratio is calculated as to the transmitted and scattered detected radiation values. When this ratio changes significantly, it is indicative that the K edge has been reached. Since the sample has a known K absorption edge, this information is then used to determine the kilovoltage level.
Whereas Harris, supra, employs a chemical element having a K absorption edge for use in determining the kilovoltage of a diagnostic X-ray generator, there is no discussion or recognition presented as to how a single pair of filters may be employed having a useful range corresponding essentially to that of the diagnostic range of from for example 40 kVp to 150 kVp. Specifically, Harris does not recognize or discuss the limited linear range or the limited dynamic range of filters employed in prior art kVp meters.